With the popularization of smart terminals such as mobile phones, computers and PDA (personal digital assistant), in order to better ensure the security of users' privacies and data stored in devices, existing smart terminals may have a screen locking function. There may be many methods for screen unlocking corresponding to this function, for example, character password unlocking, fingerprint recognition unlocking and face unlocking, etc. However, the character password unlocking may be problematic due to forgetting the password or stealing of the password by others, or unavailability of the fingerprint recognition once the finger is injured, and face unlocking may be difficult once the face becomes fat or thin. Furthermore, in the prior art, no technical solution is proposed where unlocking is achieved according to eyeprint and appropriate frames are selected according to ambient brightness or image brightness to accelerate the unlocking speed. Therefore, the prior art can be improved and developed.